Aircraft flyover noise is a well-recognized environmental problem that has received much public attention. Present local and federal regulations substantially limit the magnitude of such noise, and even more stringent regulations are under active consideration. In a continuing effort to develop new techniques for achieving aircraft noise suppression to meet and/or exceed present and proposed noise regulations, the aircraft industry has dedicated a substantial amount of effort towards acquiring a thorough understanding of the noise source mechanisms within aircraft. This necessarily requires that highly accurate and reliable aircraft noise measurements be available, particularly for aircraft while in flight.
The measurement of aircraft flyover noise is typically performed by flying an aircraft at a relatively low altitude over a ground-based microphone installation. A problem associated with such installations resides in the fact that any vertical spacing between the ground and the microphone gives rise to a condition wherein the microphone is subjected to noise signals both directly from the aircraft and from a ground reflected component of the direct signal. As a consequence, there occur reinforcements and cancellations of the total signal measured at the microphone that are a function of both noise signal frequency and the relative position of the microphone with respect to the aircraft. Ground reflection interference in flyover noise spectra is undesirable when the objective of the noise analysis is a detailed breakdown of the various components of noise produced solely by the aircraft.
The interference phenomenon resulting from the interaction of direct and reflected noise signals can be mitigated to a substantial degree by the use of a ground plane microphone. Very briefly, a ground plane microphone consists of an otherwise conventional microphone that is positioned immediately adjacent an acoustically hard surface. A characterizing feature of such a surface is that it reflects substantially all of the energy associated with incident noise signals. In other words, little, if any, of the energy is absorbed by the surface. In the ideal situation, the microphone is surrounded by an acoustically hard surface having infinite horizontal dimensions, and the microphone's diaphragm is supported such that it lies in upwardly facing, coplanar alignment with the surface. Such a microphone installation will measure aircraft flyover noise as a sound pressure that is twice the magnitude of what would otherwise be measured in the free field. This holds true at all frequencies and incidence angles of interest.
An excellent example of an acoustically hard surface is an aircraft runway. A ground plane microphone that closely simulates ideal conditions is achieved by mounting a microphone in the runway such that the microphone's diaphragm is flush with the runway surface and surrounded on all sides by a relatively broad horizontal expanse of runway surface. While near ideal results can be achieved with such an installation, it is impractical in most applications because it requires boring a hole into the runway to accommodate the microphone. Even if providing holes in the runway were not a problem, such a microphone installation has the disadvantage of preventing, or at least making difficult, the repositioning of the microphone.
Variants of the idealized arrangement include simply lying a microphone on an acoustically hard surface such that the plane of its diaphragm is perpendicular to the surface and parallel to the aircraft flight path. This is a so-called grazing incidence microphone. Because the microphone diaphragm is very close to the ground, and because the direct and reflected noise signals are almost in phase at low and mid-frequencies, a pressure doubling effect similar to the idealized arrangement is obtained at these frequencies. The main disadvantage of this arrangement is that the finite vertical dimension of the microphone diaphragm is still such that undesirable interference between direct and reflected noise signals occurs at the microphone, particularly at the higher frequency end of the aircraft flyover noise spectrum.
Another variant employs a microphone that is suspended in facing alignment with an acoustically hard surface, but spaced vertically therefrom. This arrangement is known as an inverted microphone, and its response is very similar to that of a grazing incidence microphone. It likewise suffers from undesirable interference between direct and reflected noise signals at the microphone.
While grazing incidence microphones and inverted microphones have proven useful for measuring aircraft flyover noise to a more accurate degree than microphones suspended above ground, they are nevertheless still subject to the aforementioned interference effects that detract from their ability to accurately measure noise over the entire frequency spectrum of interest.
A ground plane microphone that substantially avoids the problems associated with interfering noise signals, while enjoying much greater versatility than the near idealized arrangement previously discussed, comprises a microphone having its diaphragm flush-mounted with the upper surface of a relatively lightweight, acoustically hard plate that is dimensioned to reflect all noise signals falling within the frequency spectrum of interest. This arrangement is known as a flush-dish microphone, and generally takes the form of a circular disc having the microphone diaphragm located at the disc center. A flush-dish microphone closely approximates an ideal ground plane microphone in that it is substantially free of the undesirable interference effects previously noted for most noise signal angles of incidence. One drawback associated with flush-dish microphones resides in the fact that the noise signal at the microphone diaphragm is contaminated by signals diffracted by the periphery of the disc whenever the aircraft is located substantially directly overhead the microphone. Thus, wile flush-dish microphones have proven useful for obtaining highly accurate noise signal measurements, they too suffer from interference effects, albeit to a lesser degree than other ground plane microphones.